Why do sharp ends of blades attract?

[PhysicoRaj]

Hi everyone..
I was doing some experiments on diffraction and interference. For the single slit I used a shaving razor blade and cut it into half along its length. Then I placed the two halves together forming a slit such that the two sharp edges almost touched each other, similar to the one in the attachment.
Then I shone my laser and got some diffraction patterns..
I repeated the experiment with the razor blades, not on any mount, but poked onto a rubber cube so that I could change the aperture during the experiment. As I started moving the blades together, they simply attracted each other and got stuck, leaving no aperture. I tried again, at a particular distance they stick to each other. But with the blunt edges facing each other, this does not happen. It seems they are attracting each other.. why? This seems to be a new phenomenon for me, so let me know what's behind all this..
Thanks for any replies.

 

[mrspeedybob]

Any chance that the blades are slightly magnetized? The magnetic field would be more concentrated by the sharp edge.

 

[PhysicoRaj]

Any chance that the blades are slightly magnetized? The magnetic field would be more concentrated by the sharp edge.

I don't think so.. it was a new shining blade and was unused. It was kept in the bathroom where no magnetic substances or magnets are kept...

 

[CWatters]

It might have become magnitised during manufacture (eg sharpening)?

 

[HallsofIvy]

If it was a "new shining blade and was unused" then it is more likely to be slightly magnetised. The sharpening process can align molecules and give a slight magnetism to the blade.

 

[Danger]

Are they close enough together that van der Waals force might play a part?

 

[ifconfig]

Are they coated in any material which may be magnetized ?? The fact they are attracting (or repelling if the case was made) really suggests magnetization.. Interesting though.. Have you tried with other brands of blade ?

 

[lightarrow]

As I started moving the blades together, they simply attracted each other and got stuck, leaving no aperture. I tried again, at a particular distance they stick to each other. But with the blunt edges facing each other, this does not happen. It seems they are attracting each other.. why?

Sorry for the naive question, but are you sure they are really attracting each other and it's not the optical effect due to diffraction (as if you look at two fingers approaching each other while in front of the eye)?

 

[ifconfig]

Sorry for the naive question, but are you sure they are really attracting each other and it's not the optical effect due to diffraction (as if you look at two fingers approaching each other while in front of the eye)?

I took the assumption this was based off an experiment which was contained, also repeated .. Given the layout there are grounds you are right, however its hard to say.The full extent of the experiment would need to be explained. For example was there any liquid in the container for which they transported etc.

But based off your comment I can take the assumption(although never should be taken) that because it was repeated it was not the diffraction effect. I could very well be wrong!

 

[a1call]

The same way that a microwave oven's mesh screen prevents wavelengths larger than the holes through. The light through the slit will be blocked as soon as the distance between the blades falls below the wavelength of the visible light.
That would be at about 0.0004 mm.

 

[sophiecentaur]

How much force was involved in this "attraction"? Was it actually measurable?
If the effect was only 'seen' and not 'felt' then the diffraction explanation seems, to me, most likely. Except that it wasn't reported for a pair of blunt edges. You can get a similar (optical) effect as you bring finger and thumb (very blunt) together. As they are brought closer and closer together, a point is reached where the diffraction effect gives an apparent 'jump' and the gap appears to close up.
This effect (and also the visible fringes) is difficult to record on a photograph because the aperture of the eye is less than that of a camera. But it does work if you use a pinhole to reduce the aperture.

 

[ifconfig]

How much force was involved in this "attraction"? Was it actually measurable?
If the effect was only 'seen' and not 'felt' then the diffraction explanation seems, to me, most likely. Except that it wasn't reported for a pair of blunt edges. You can get a similar (optical) effect as you bring finger and thumb (very blunt) together. As they are brought closer and closer together, a point is reached where the diffraction effect gives an apparent 'jump' and the gap appears to close up.
This effect (and also the visible fringes) is difficult to record on a photograph because the aperture of the eye is less than that of a camera. But it does work if you use a pinhole to reduce the aperture.

You are offering suggestions based on the light issue. This may have already (and most likely) taken care of.

(You can get a similar (optical) effect as you bring finger and thumb (very blunt) together.) You can, but demonstrated twice.. In single eye case, this is completely void.

 

[NascentOxygen]

As I started moving the blades together, they simply attracted each other and got stuck, leaving no aperture. I tried again, at a particular distance they stick to each other. But with the blunt edges facing each other, this does not happen. It seems they are attracting each other.. why?

Thought experiments that come to mind:

Maybe try various half-blade combinations, to see whether there is always attraction, or is there sometimes repulsion?

Make a sensitive magnetometer and test the blades for N's and S's.

It may be possible to put them through a demagnetiser, and see whether the effect is diminished? A cheap demagnetiser may not be up to the task, though.

Perhaps you could slightly magnetise one of them and see whether you can effect repulsion? May need to try this twice, changing the polarity.

Cut a half-blade in half, trying not to subject it to shock. The pieces should give you a pair of similar magnets. Do these quarter blades still show attraction or do they now repel?

 

[Borek]

I don't think so.. it was a new shining blade and was unused. It was kept in the bathroom where no magnetic substances or magnets are kept...

Unless you live on the Moon, your bath is right on the magnet surface.

 

[sophiecentaur]

You are offering suggestions based on the light issue. This may have already (and most likely) taken care of.

(You can get a similar (optical) effect as you bring finger and thumb (very blunt) together.) You can, but demonstrated twice.. In single eye case, this is completely void.

But there appears to be an anomaly, if the experiment was carried out right and if the effect was optical. This anomaly needs an explanation, which could be to do with the observation method. I was not just repeating old ideas.
Perhps you could read more carefully before complaining so early?

 

[B0b-A]

Hammering iron makes it into a magnet

Hammering iron can cause it to become magnetised ...

Take a steel bar, hold it vertically, and strike the end several times with a hammer, and it will become a permanent magnet.

http://www.princeton.edu/ssp/joseph-henry-project/permanent-magnet/

The manufacturing process would have involved applying hammer-like forces the steel to make the razor blades ... http://en.wikipedia.org/wiki/Rolling_(metalworking)#Cold_rolling

 

[sophiecentaur]

So far, there are no actual quoted measurements of force so the conversation isn't really going anywhere, is it? If there really is a magnetic force involved then hanging up one blade on a light thread and bringing the other blade close will reveal it. This would mean a result for the thread and put an end to speculation.

 

[PhysicoRaj]

Sorry for the naive question, but are you sure they are really attracting each other and it's not the optical effect due to diffraction (as if you look at two fingers approaching each other while in front of the eye)?

Yeah, I'm sure they attract, because at the time I stop getting any light on the screen, the rubber cubes I poked the blades into, tilt towards the middle.

 

[PhysicoRaj]

Are they coated in any material which may be magnetized ?? The fact they are attracting (or repelling if the case was made) really suggests magnetization.. Interesting though.. Have you tried with other brands of blade ?

This was 'Wilkinson sword stainless steel'...

 

[PhysicoRaj]

Unless you live on the Moon, your bath is right on the magnet surface.

AAhhh.. for that matter,.. the blunt ends don't show this effect! Is it because the intensity is more at sharp edges??

 

[PhysicoRaj]

So far, there are no actual quoted measurements of force so the conversation isn't really going anywhere, is it? If there really is a magnetic force involved then hanging up one blade on a light thread and bringing the other blade close will reveal it. This would mean a result for the thread and put an end to speculation.

You can get an idea of the force if I were to say that the two rubber cubes into which I poked the blades(one blade for one cube) actually tilted towards themselves, while the blades tried to attract each other. By the way, this proves it's not an optical illusion.

 

[PhysicoRaj]

And..

Cut a half-blade in half, trying not to subject it to shock. The pieces should give you a pair of similar magnets. Do these quarter blades still show attraction or do they now repel?

Yeah I tried this and the answer is they 'attract'.
I also tried using a new blade and this one shows this effect too.

 

[PhysicoRaj]

It might have become magnitised during manufacture (eg sharpening)?

If it was a "new shining blade and was unused" then it is more likely to be slightly magnetised. The sharpening process can align molecules and give a slight magnetism to the blade.

Yeah this seems likely to me.

 

[sophiecentaur]

You can get an idea of the force if I were to say that the two rubber cubes into which I poked the blades(one blade for one cube) actually tilted towards themselves, while the blades tried to attract each other. By the way, this proves it's not an optical illusion.

Well that's an important contribution to the discussion. If you're really sure about that evidence, we can now rule out the diffraction solution.
However, there is one remaining problem and that is the fact that you would really need to show some force of repulsion to prove that there has been permanent magnetisation. This is the acid test.
There are two basic ways that the blades could have been magnetised and we have to assume they were made identically . Either there the N and S poles lie along the length of the cutting edge or one pole is on the back and one is on the sharp edge.
If the N and S poles were arranged 'front to back' (blunt edge to sharp edge) then the blades, when brought sharp edge to sharp edge would repel - N against N or S against S - so this implies that N and S lie in line with the cutting edge. Bringing the sharp edges together would bring S of one to N of the other (=attraction) OR, if you rotate one blade, that would bring N to N and S to S (= repulsion). If you cannot get repulsion with some orientation then I don't see how the effect can be because the blades are magnetised. You could be looking at capillary attraction if the blades are wet or electric attraction involving charges stored because of the rubber insulation.
Any chance of a photograph of it happening? Have you tried a selection of blades / combinations and do you always get attraction?

 

[PhysicoRaj]

Any chance of a photograph of it happening?

I'll try to take one. I have tried to film this before. It takes a video (and of good resolution) to see that. It's difficult because additional diffraction effects and illusions curtain us from deciding what's happening in the video. It's especially great to feel that in your hands... as I move the rubber cubes inwards, after some distance I feel the blades moving without my help. Then keen observations reveal the cubes tilt towards themselves at that moment by a small angle. With a sharp edge of one blade facing the other sharp edge of the same blade, you can even feel the small discomfort in separating the blades after they stick!

Have you tried a selection of blades / combinations and do you always get attraction?

Yes. I first took two separate blades and observed this effect when the sharp edges of the two separate blades were brought together. They attract. This holds good for the two sharp edges of the same blade also, i.e.,cutting a blade along it's length into half. A blunt and a sharp edge may produce this effect(I'm not sure because it was too difficult to decide. It was almost like attracting but not attracting). A blunt and blunt edge does not seem to produce considerable effect(it was nil in my experiment).

 

[sophiecentaur]

If you only ever get attraction then I don't see how it can be permanent magnetism.

 

[PhysicoRaj]

If you only ever get attraction then I don't see how it can be permanent magnetism.

Yes. That's what I also think. To decide this today I made another experiment. I marked the two different 'whole' blades as A and B. The different edges as XA, XB, YA, YB... like that.
→With the one sharp edge of A facing one sharp edge of B, I got a strong attraction.
→With the same edge of A facing the other sharp edge of B,I still get attraction.
Now I cut them longitudinally into half.
→Two blunt ends do not attract. They neither repel.
→A blunt and sharp edge show slight attraction.
→With the same sharp edge and other sharp edge, I really had to pull them apart to separate them!
→With one sharp edge of split blade facing a sharp edge of a whole blade, I feel there was 'some' repulsion. I am not sure but I say it because the other blade when moved laterally(like a flyby) in front of the sharp edge of whole blade, the sharp edge, moved, but in the direction opposite to the position of the moving blade.
So, is it that one half of the blade is magnetised in a manner opposite to the manner in which the other is? I just couldn't identify poles. And the sharp edges attract more in case of a half blade than a whole blade.

 

[sophiecentaur]

Do you have any iron filings? They may give a clue to any magnetic field pattern. In fact, you could try a blade and another, steel, object and see if there's any force there.
Failing this, perhaps you can say that you've found another 'Force' to add to the list of accepted ones.

 

[PhysicoRaj]

Failing this, perhaps you can say that you've found another 'Force' to add to the list of accepted ones.

Oh.. that's light years away from me because I know it's not. Why? Read on..

Do you have any iron filings? They may give a clue to any magnetic field pattern. In fact, you could try a blade and another, steel, object and see if there's any force there.

I have done the ultimate test and I believe it's the magnetism behind the blades' behavior. The only instrument I have, to test magnetism on such a small scale is my magnetic compass. Using that I could map the unusual magnetization in the blade. It is magnetised along its length, parallel to the edges, but not ideally parallel. The direction of magnetisation is approximately 15° to 20° inclined with a line along its length. This might be the reason for the weird behaviour between the two blades. This means one end of the sharp edge acts as a strong north pole while the other of the same edge as a weak south pole. So when you cut it into half longitudinally, the south pole comes to the blunt edge, so two blunt edges do not show any attraction. May be the blunt edges don't have enough mag. intensity to show repulsion. So, actually the lateral inversion of two whole blades whose sharp end is facing the same of the other blade would have no effect if the blades are not upside down!

 

[sophiecentaur]

Oh.. that's light years away from me because I know it's not. Why? Read on..


I have done the ultimate test and I believe it's the magnetism behind the blades' behavior. The only instrument I have, to test magnetism on such a small scale is my magnetic compass. Using that I could map the unusual magnetization in the blade. It is magnetised along its length, parallel to the edges, but not ideally parallel. The direction of magnetisation is approximately 15° to 20° inclined with a line along its length. This might be the reason for the weird behaviour between the two blades. This means one end of the sharp edge acts as a strong north pole while the other of the same edge as a weak south pole. So when you cut it into half longitudinally, the south pole comes to the blunt edge, so two blunt edges do not show any attraction. May be the blunt edges don't have enough mag. intensity to show repulsion. So, actually the lateral inversion of two whole blades whose sharp end is facing the same of the other blade would have no effect if the blades are not upside down!

You can take the two halves, roast them in a flame and then see if they show the same effect. That would indicate whether or not it's magnetic because the heating should destroy - or at least modify - their magnetisation.

 

[PhysicoRaj]

Good idea! But now I'm quite sure it's magnetism. The compass deflects.

 

[sophiecentaur]

Good idea! But now I'm quite sure it's magnetism. The compass deflects.

To be certain, you would need to eliminate or change the magnetisation and see if the force alters.

 

[PhysicoRaj]

Confirmed. I heated them to red hot, and now they are just steel blades. No magnetism.
Maybe they didn't show repulsion because the repelling end was blunt and B was not intense enough.
Thanks for all who helped me in understanding this.

BTW, Why do metals on heating show permanent colouration? like this:

 

[B0b-A]

BTW, Why do metals on heating show permanent colouration? like this: ...

en.wikipedia.org/wiki/Bluing_(steel)

 

[PhysicoRaj]

en.wikipedia.org/wiki/Bluing_(steel)

Thanks